The present invention relates generally to devices and methods for determining the concentration of alcohol in a mixture of gases. More particularly, the invention relates to a device and method for determining the concentration of alcohol in a breath sample for application in sobriety detection systems and sobriety interlock systems for vehicles and other machines.
Various techniques have been employed for calculating a person's blood alcohol concentration by measuring breath samples. In a first method, the alcohol content in a breath sample is measured using a semiconductor sensor commonly referred to as a Taguchi cell. Although this method provides a low cost device, instruments incorporating this method have proved to have poor accuracy.
A second method employs an infrared absorption technique for determining the blood alcohol concentration. This method has proven to have very high accuracy levels. However, the sensor systems incorporating this technique are cost prohibitive for many applications.
A third method employs a fuel cell together with an electronic circuit. This method is described in U.S. Pat. No. 4,487,055. Although this method allows for more accuracy than the first method, the systems employing this method have continued to be relatively expensive. One reason for the high cost associated with the fuel cell techniques is that the method requires that the breath sample be of a determinable volume. Historically, this has been accomplished through the use of positive displacement components such as piston-cylinder or diaphragm mechanisms. The incorporation of such components within an electronic device necessarily increases the costs associated with the device.
The current invention departs from the use of the expensive positive displacement mechanisms while retaining the accuracy provided by the fuel cell. The current invention employs a valve controlled by a computing device such as a microprocessor to regulate the total volume of gas passing through the fuel cell.